Cells and Batteries

Chemical cells and batteries convert chemical energy into electrical energy through chemical reactions. This lesson covers how cells work, the difference between cells and batteries, the factors that affect the voltage of a cell, and how these link to the Energy Changes topic in AQA GCSE Chemistry.

---

What Is a Chemical Cell?

A chemical cell (also called an electrochemical cell or voltaic cell) is a device that produces a voltage (potential difference) and an electrical current from chemical reactions. The chemicals in the cell react together, and the energy stored in their chemical bonds is converted into electrical energy.

A basic chemical cell contains:

• Two electrodes made from different metals (or one metal and one non-metal such as carbon).
• An electrolyte — a solution or paste that conducts electricity and allows ions to flow between the electrodes.

Component	Role
Electrode 1 (more reactive metal)	Acts as the negative terminal; atoms lose electrons (oxidation)
Electrode 2 (less reactive metal)	Acts as the positive terminal; ions gain electrons (reduction)
Electrolyte	Conducts ions between the electrodes, completing the internal circuit
External wire	Carries electrons from the negative to the positive electrode (electrical current)

Exam Tip: Current flows from the positive terminal to the negative terminal in the external circuit (conventional current), but electrons actually flow from negative to positive. In chemistry, focus on the electron flow.

---

How Does a Chemical Cell Work?

When two different metals are placed in an electrolyte and connected by a wire:

1. The more reactive metal (higher in the reactivity series) loses electrons more readily. These electrons flow through the external wire to the less reactive metal.
2. The flow of electrons through the wire is an electric current.
3. Inside the cell, ions in the electrolyte move between the electrodes to complete the circuit.
4. The cell will produce a voltage until one of the reactants is used up.

graph LR
    A["More Reactive Metal (e.g. Zinc) - Negative Electrode"] -->|"Electrons flow through wire"| B["Less Reactive Metal (e.g. Copper) - Positive Electrode"]
    C["Electrolyte (e.g. dilute sulfuric acid)"] -->|"Ions carry charge internally"| A
    C -->|"Ions carry charge internally"| B
    B -->|"Complete circuit"| A

---

What Affects the Voltage of a Cell?

The voltage (potential difference) produced by a chemical cell depends on two main factors:

Factor	Effect on Voltage
The metals used for the electrodes	The greater the difference in reactivity between the two metals, the higher the voltage
The electrolyte used	Different electrolytes can affect the voltage produced

Reactivity and Voltage

The reactivity series helps predict which combination of metals will produce the highest voltage:

Metal	Reactivity
Potassium	Most reactive
Sodium
Calcium
Magnesium
Aluminium
Zinc
Iron
Tin
Lead
Copper
Silver
Gold	Least reactive

A cell made from magnesium and copper will produce a higher voltage than one made from zinc and copper, because the difference in reactivity between magnesium and copper is greater.

Exam Tip: The bigger the gap between the two metals in the reactivity series, the bigger the voltage. If asked to choose metals for the highest voltage, pick one from near the top and one from near the bottom of the reactivity series.

---

Cells vs Batteries

Term	Definition
Cell	A single electrochemical unit that produces a voltage from a chemical reaction
Battery	Two or more cells connected together in series to provide a higher total voltage

The word "battery" is commonly used in everyday language to describe a single cell (e.g., an AA battery), but in chemistry the correct term for a single unit is a cell. A battery is technically a combination of cells.

When cells are connected in series (end to end), their voltages add together:

• 1 cell at 1.5 V = 1.5 V total
• 2 cells at 1.5 V = 3.0 V total
• 4 cells at 1.5 V = 6.0 V total

---

Non-Rechargeable and Rechargeable Cells

Feature	Non-Rechargeable	Rechargeable
Also called	Primary cells	Secondary cells
Chemical reaction	Irreversible — once reactants are used up, the cell is dead	Reversible — passing a current through the cell reverses the reaction
Lifetime	Used once and disposed of	Can be recharged and reused many times
Cost	Cheaper to buy initially	More expensive to buy, but cheaper over time
Environmental impact	Creates more waste; must be recycled or disposed of properly	Less waste, but manufacturing has environmental costs
Examples	Alkaline cells (AA, AAA), zinc-carbon cells	Lithium-ion, nickel-metal hydride (NiMH), lead-acid

Advantages and Disadvantages of Rechargeable Cells

Advantages	Disadvantages
Can be reused many times, reducing waste	More expensive initial purchase
Lower long-term cost	Lose capacity over many charge cycles
Better for the environment over time	Require a charger (uses electricity)
Higher energy density in many designs	Some types have memory effect issues

graph TD
    A["Chemical Cell"] --> B["Non-Rechargeable (Primary)"]
    A --> C["Rechargeable (Secondary)"]
    B --> D["Irreversible reaction"]
    B --> E["Disposed after use"]
    C --> F["Reversible reaction"]
    C --> G["Can be recharged"]
    G --> H["External current reverses the chemical reaction"]

Exam Tip: When comparing rechargeable and non-rechargeable cells, make sure you mention BOTH advantages AND disadvantages of each. Questions often ask for a balanced evaluation.

---

The Simple Lemon Cell Demonstration

A simple cell can be made by pushing two different metal electrodes (e.g., a zinc nail and a copper coin) into a lemon. The acidic juice of the lemon acts as the electrolyte.